Point defects can greatly influence the properties and functionality of a material in electronics, photovoltaics, and catalysis applications. One important parameter influencing defect concentration and charge state is doping, which can occur both intentionally and unintentionally. The effects of doping can be local and global. The local effects occur due to a local change in electronic structure and lattice relaxation around the dopant, while the global effects include creation of a Fermi level, formation of a space-charge region, and band bending at the surface. Experiments measuring defect charge states and concentrations as a function of thermodynamic variables (T, p, doping) are scarce. Previous theoretical approaches often aim at a description of isolated defects, or, more recently, dopant-defect complexes. In this talk, I address the challenges for ab initio modeling of point defects in different charge states at metal-oxide surfaces, using as an example oxygen vacancies (F centers) in MgO. Aspects that are considered include realistic modeling of defect charge compensation in doped materials, and how defect formation energies can be determined using ab initio atomistic thermodynamics in combination with hybrid density-functional theory (DFT), with parameters of the exchange-correlation functional optimized according to a condition on DFT ionization energies. Further, I discuss how the standard methodology for calculating defect formation energies can be extended to include the electrostatic free energy due to charge transfer between defects and dopants.
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